The aim of the study is a comparison of inducible and non-inducible mouse 3T3 cells transformed by a thermosensitive mutants, ts-a, of polyoma virus. Two questions are presently under investigation: (1) Are the virus-specific RNA species transcribed in inducuble ts-a 3T3 cells and their non-inducible derivatives the same or do they differ from each other? (2) Are the viral sequences which are integrated into the host DNA identical in inducible and non-inducible cells, and are the sites of integration into the cellular DNA the same in both cases? Hybridization experiments between cellular RNA of inducible or non- inducible cells and separated strands from polyoma DNA fragments obtained after H. parainfluenzae enzyme II digestion are in progress and should answer question (1). The representation of different Hpa II-polyoma fragments in the cellular DNA of inducible and non-inducible cells will be compared in reassociation kinetics studies (question 2). In addition, the fragment pattern obtained by RI-digestion of cell DNA from inducible and non- inducible cells will be analyzed with special emphasis on the fragments containing both cellular and viral sequences.